As is known, the production of integrated devices envisages testing steps that allow to check proper operation of the components obtained. Tests of this kind, such as, for example, EWS (Electrical Wafer Sorting) or WS (Wafer Sorting), are normally performed at the level of semiconductor wafer, by coupling ATE (Automatic Test Equipment) to contact pads of the DUTs (Devices Under Test) present on the wafer itself. The pads can be specifically dedicated to the tests, but more frequently a portion of the entire set of the pads of the devices that will subsequently have to serve for the coupling of the devices to enable their use in the final application are also employed for testing.
For coupling the testing machine to the wafer under test, interface cards or “probe-cards” are generally used, that normally include a PCB (Printed Circuit Board) provided with probe-electrodes, which is placed in contact with the pads with a given pressure to ensure electrical coupling in all cases.
After the electrical testing steps, the wafers, whether single or composite, are cut into dice or chips and assembled in packaging structures. Then, the pads are electrically coupled to pins of the packaging structures for coupling with external devices. Electrical coupling is frequently obtained using a wire-bonding technique, with which wire couplings between the pads and the pins are provided, or else bumps can be used.
The same pads must hence serve both for testing and for electrical coupling to the outside world.
The mechanical action of the probe-electrodes causes, however, surface damage to the pads, which can deteriorate the quality of the electrical couplings obtainable during assembly up to the point of rendering the pads themselves unusable. Amongst other things, in various cases a single testing sequence is not sufficient, and various machines are used to execute different test sequences. This may entail positioning the interface card or cards a number of times, thus increasing the risk of damage to the pads.
To overcome this drawback, various solutions have been proposed, none of which is, however, altogether satisfactory.
For example, in U.S. Pat. No. 6,844,631, which is incorporated by reference, it has been proposed to provide auxiliary metallizations, which extend not only on the pads but also on part of the passivation layer surrounding the pads. In this way, a part of the auxiliary metallization (for example, on the passivation layer) can be dedicated exclusively to contacting in the testing step. The remaining part of the auxiliary metallization, which does not come into contact with the probe-electrodes, is not damaged, and can be used to obtain a high-quality electrical coupling via wire-bonding or some other technique. Alternatively, the auxiliary metallization can be removed chemically after the testing step, leaving the pads free and without damage.
If the quality of the contacts can in this way be preserved, it has, however, been noted that the regions underlying the metallizations around the pads are subject to failure, especially when metal lines made of copper are present, which may be exposed to the oxidizing action of air. Moreover, the area provided for each pad typically is increased to enable the production of the auxiliary metallizations. The density of the pads is hence disadvantageously reduced.
Other variants make up in part for the limits of the solution described, but entail numerous additional processing steps, which are rather complex and lead to a significant increase in the unit cost of production of the devices.
According to a different solution described in U.S. Pat. No. 5,391,516, which is incorporated by reference, the contact pads are repaired after the testing step using a laser beam that heats the pads themselves beyond the melting temperature (e.g., for aluminium 660° C.). Leaving aside the evident effects of a possible poor alignment of the laser beam, the extremely marked thermal gradient produced by concentrated heating can easily cause failure owing to non-uniform thermal expansion.